The RoundedCorners extender applies rounded corners to existing elements. To accomplish
this it inserts elements before and after the element that is selected, so the overall
height of the element will change slightly. You can choose the corners of the target
panel that should be rounded by setting the Corners property on the extender to None,
TopLeft, TopRight, BottomRight, BottomLeft, Top, Right, Bottom, Left, or All.

RoundedCorners Properties

The control above is initialized with this code. The italic properties are optional.